1. Field of the Invention
The present invention relates to a blade-like connecting needle for use on a probe card as a probing equipment or a contact probe for connecting a measuring unit to a semiconductor wafer for making measurement on the semiconductor wafer, and more particularly to a blade-like connecting needle for use on a probe card, which is capable of stably and greatly reducing a measurement waiting time that serves as an important factor in measuring a small current.
2. Description of the Related Art
One conventional probe card is shown in the plan in view of FIG. 10 among the accompanying drawings, and a portion of the conventional probe card for connection to a semiconductor wafer is shown at an enlarged scale in FIG. 11. A current measurement signal electrode and a guard electrode which extend from a measuring unit are connected downwardly to respective pads A and B shown in FIG. 11 via pogo pins. The pad A is electrically connected to pads F and G, but insulated from the pad B, which is positioned outwardly of the pad A. A signal line connected to the pad A extends via a cable to a needle mount area D, where it is connected to a probe needle of certain shape. The probe needle is connected to desired electrodes on a semiconductor wafer. The needle mount area D generally has single-core needles, coaxial needles, or blade-like connecting needles. FIGS. 12 through 15 of the accompanying drawings show conventional blade-like connecting needles at an enlarged scale in the needle mount area D on the probe card shown in FIG. 11. FIGS. 14 and 15 show in perspective one of the conventional blade-like connecting needles as viewed from respective opposite sides. Signal lines 114 (FIG. 12) from the measuring unit are connected to blade signal lines 111 shown in FIG. 15, which are connected to signal patterns 120 on tip ends of the blade-like connecting needles. The signal patterns 120 are connected by soldering to respective probes 116 that are held in contact with desired electrodes on a semiconductor wafer. In FIGS. 14 and 15, guard patterns 112 (112a, 112b, 112c) are disposed to cover substantially entirely two faces 112b, 112c and a portion of one face 112a of the blade-like connecting needle. The guard pattern 112b, which may be viewed as a bottom surface of the blade-like connecting needle, is fixed by soldering to the needle mount area guard pattern 113 (FIG. 12) on a probe card 117. The blade-like connecting needles are held in one-to-one correspondence to channels of the measuring unit. If counter measures against problems associated with small currents and noise are required, the patterns 112 are provided between the blade signal lines 111 and another potential, and connected to the respective pads B (FIG. 11). The guard patterns 112 may extend from one side of a blade body 115 which is made of an insulating material, on which the signal line 111 is placed, to an opposite side of the blade body 115. The guard patterns 112 serve to prevent a current from flowing from or into the signal lines 111 to another potential via the surface of the probe card substrate insulator 117, for example.
Preferably, an active guard whose potential is maintained at the same level as the potential of the signal line 111 is usually used as the guard, no signal current leaks from or into the signal line via the guard patterns to or from another potential. Any dielectric absorption due to the insulator between the signal line and the guard does not occur because the potential difference is nil. Therefore, the measurement of a small current can instantaneously be started when a certain voltage is applied to the signal line.
Even when a passive guard for fixing the guard potential to a certain fixed potential at all times is employed, some advantageous effect can be expected. As shown in FIG. 12, as xe2x80x9canother patnetialxe2x80x9d to a particular signal line, the lower guard pattern belonging to an adjacent signal line channel, and, in addition, many signal lines, guard patterns, and patterns providing other potentials are present at a high density.
As can be seen from FIG. 12, the signal line 111 is disposed and exposed on the surface of the blade body 115 in the conventional structure. If a small current on the order of fA is to be measured, a measurement waiting time is long because of a dielectric absorption current caused by the insulator between the signal line and the other potential pattern having a different potential. In the presence of a capacitive coupling to other potential patterns, a longer measurement waiting time is needed due to the time required to charge the capacitance. The capacitive coupling should desirably be eliminated in view of possible fluctuations of other potentials and a possible superposition of noise.
Japanese laid-open patent publication No. 8-330369 discloses an attempt to place the needle mount area D in a lower trenched position for preventing different potential present in other parts of the system from being directly xe2x80x9cseenxe2x80x9d by the signal line, and to position guard patterns on side faces of the trench. The disclosed attempt provides good results. However, since the disclosed attempt requires complex processing on the probe card substrate insulator 117, it is difficult and costly to produce a high-density blade package.
The above characteristics will be described on the basis of an equivalent circuit. An equivalent circuit of the structure shown in FIG. 12 is illustrated in FIG. 16. For the sake of brevity, a dielectric absorption term is modeled by a linear approximation. As also shown in FIG. 15, a signal line terminal 60 in FIG. 16 corresponds to the signal line 111, a guard terminal 62 to the guard patterns 112 (112a, 112b, 112c), and a terminal 64 having a different potential to an adjacent channel pattern. For an active guard, since the signal line terminal 60 and the guard terminal 62 are always held at the same potential, they may be considered as being short-circuited. Thus, only the other potential terminal 64 corresponding to the adjacent channel pattern may be considered from the terminal 60. It is assumed that the terminal 64 of differing potential has a certain constant potential and the potential of the signal terminal 60 changes stepwise with respect to the potential of terminal 64. Since the charging and discharging of a capacitive component C3 is finished instantaneously, it does not pose any problem on a settling time. However, the time constant of a resistive component R4 and a capacitive component C4, which represents a dielectric absorption term, generally shows a large value of several tens of seconds, it takes a very long period of time to measure a small current until an error current value falls into an allowable range. For example, when the terminal voltage of the conventional blade changes by 10 V, it occasionally takes several tens of seconds until the error current value due to the dielectric absorption term falls into a range of the order of fA. Required settling times differ greatly depending on the position of the blade-like connecting needle and the manufacturing lot thereof because of variations in insulator materials. In addition, since the insulator characteristics greatly vary depending on the environmental conditions (temperature and humidity), pollution, etc., the insulator may have poor weather resistance and poor environment resistance. The conventional probe card (a combination of a probe card blank board, wiring cables, and blade-like connecting needles) has been problematic because its specification needs to have a large margin against material variations and environmental conditions. Japanese patent application No. 2000-036636 (Japanese laid-open patent specification No. 2001-231195) reveals an invention relating to a probe card blank board that is improved to solve the above problems and provide substantially ideal characteristics. Therefore, blade-like connecting needles still remain to be improved.
One known solution is to use a coaxial needle instead of a blade-like connecting needle. Using a coaxial needle structure can solve the conventional problem of the large settling time because the signal line is covered with the guard.
However, the coaxial needle which uses a good-quality insulator, typically polytetrafluoroethylene (PTFE), is of poor durability and is highly expensive. Since the durability of the coaxial needle is much lower than other connecting needles, the downtime of the device due to maintenance and repair is longer, resulting in a poorer cost balance. In order to improve the durability and reduce costs, attempts have been made to improve the insulator. However, if no guard can be used, or if a passive guard is used, or if such a use occur prior to the measurement of a small current, then it takes a very long period of time until the signal is settled.
In order to solve the above problems, there is provided in accordance with the present invention a blade-like connecting needle for measuring a semiconductor wafer, comprising a blade signal line for transmitting signal from the semiconductor wafer, a support insulator covering at least a portion of the blade signal line, a plurality of blade guard patterns disposed in or on the support insulator for electromagnetically shielding the blade signal line, and a probe supported on the support insulator and connected to the blade signal line. At least a portion of the probe may be covered with an insulating material. The blade-like connecting needle may further include a connector for connecting the blade signal line to an external circuit. The connector may include a signal line covered with an insulating material, in at least a portion thereof. The blade-like connecting needle thus constructed is inexpensive, and has an increased capability for measuring a small current and also has stable characteristics.